This invention relates to helical gears and more particularly to helical gears in combination with spur or straight toothed gears for purposes of eliminating axial force and consequently eliminating the need for thrust bearings.
As is well known and recognized by those skilled in the art, a pair of helical gears can be used to transfer power or force from one parallel shaft to another while providing considerable advantages over similar spur or straight toothed gears. For example, in regular spur gears the teeth come into contact along the entire face width and the contact is along the line parallel to the axis at all times. Parallel helical gears start contact at a point along one edge of the tooth and the contact progresses across the tooth width to the other edge of the tooth. The teeth therefore assume the load gradually, resulting in less impact and smoother operation. This gradual assumption of the load also results in the helical gear having greater strength than that of regular spur gears. As a result, helical gears can be operated at higher speeds or greater loads than equivalent spur gears, are quieter in operation and have less vibration. Because of these advantages, helical gears are preferred for use in heavy power transmission applications, heavy duty gear boxes and the like. Unfortunately, the use of helical gears has disadvantages as well as advantages. Since the force transmitted between the teeth of two meshing helical gears is always normal to the tooth surfaces, helical gears generate a component of force along the axis of the gear which causes end thrust. In presently available gearing applications, such end thrust is typically taken care of by the use of thrust bearings, and sometimes by the use of herring bone gears. Herring bone gears, as will be appreciated by those skilled in the art, comprise two helical gears of opposite hand (i.e. the direction of the helical is different for these two gears) mounted to each shaft. Thus, the axial or end thrust created by one-half of the herring bone gear is countered by the end thrust or axial force created by the helical gear having the opposite hand. Examples of herring bone gears are illustrated in U.S. Pat. No. 415,044 issued to W. Joslin on Nov. 12, 1889 and U.S Pat. No. 1,989,663 issued to J. Bethune on Feb. 5, 1935. Further discussions concerning herring bone gears and helical gears may be found on pages 427 through 432 of a book entitled "Kinematics and Dynamics of Machinery" by Robert L. Maxwell. This book was published in 1960 by Printice Hall Inc., Englewoods Cliffs, N.J. Additional information concerning herring bone gears and other helical gears may be found at pages 397 through 405 in a book entitled "Design of Machine Elements". This book was written by Virgel M. Faires and was published in 1965 by McMillan Company, New York, N.Y.
Unfortunately, end thrust bearings may not be suitable and cannot be used in all applications. Also, of course, end thrust bearings place an additional expense upon the design of a gear transmission train. Similarly, herring bone gears in general are also very expensive and require accurate alignment along the shafts they connect. In addition, herring bone gears are very difficult and sometimes substantially impossible to arrange and mesh in close quarters.
To overcome the shortcomings of these and other available methods of controlling end thrust in a power transmission train, it is an object of this invention to provide a method and apparatus which eliminates end thrust in a helical gear transmission train.
It is another object of this invention to provide a method and apparatus for eliminating the need of thrust bearings in a helical gear transmission train.
It is a further object of this invention to provide inexpensive and readily maintained apparatus for eliminating end thrust in a helical gear transmission train.
It is still another object of this invention to provide a method and apparatus that eliminates the need of a thrust bearing at both ends of a shaft supporting a helical gear in a transmission train in which the helical gear may be either the driving gear or the driven gear.
To accomplish the above mentioned objects, as well as other objects, which will become evident from the following drawings and detailed description, the present invention comprises a first helical gear which is securely and rigidly attached at a selected distance, to a spur or straight toothed gear having the same pitch diameter as the helical gear. The spur gear is positioned and attached to said helical gear such that its center of rotation or axis is the same as the center of rotation of said helical gear. The combination helical and spur gear are mounted on a common shaft such that rotation of the shaft results in rotation of the gear combination and visa versa. A second helical gear which has teeth suitable for meshing with the teeth of the first helical gear, but with a pitch direction opposite to that of the first gear is mounted on a shaft parallel to the shaft of the first helical gear. The shaft of the second helical gear is located at a distance from the shaft of the first helical gear such that the helical threads of both gears are properly meshed. A second spur or straight toothed gear is attached to the second helical gear such that its center of rotation or axis is the same as the center of rotation of the second helical gear. This second spur gear has a pitch diameter equivalent to the second helical gear and is located at a distance from the second helical gear such that its teeth are engaged or meshed with the teeth of the first spur gear. The teeth of the first and second spur gears are preferably designed such that the width of the teeth is less than the gap between the teeth. Thus, in operation, when a rotational force is applied to one of the shafts such that one of the helical gear is in a driving relationship with the other helical gear an axial force between the two helical gears (which is inherent when two helical gears are in meshing and driving relationship) will be experienced. This axial force results in an axial motion which will continue until the teeth of the two spur gears come into contact. As the teeth of the spur gears come into contact, axial motion of the spur and helical gear combination will cease even though the first helical gear and spur gear combination continues to drive the second spur gear and helical gear combination. Thus it is seen, that according to the technique of this invention, end thrust is eliminated and consequently the need for thrust bearings is eliminated.